FlyVirgin744
Topic Author
Posts: 1282
Joined: Tue Jul 27, 1999 8:35 am

Calculus Help!

Fri Oct 12, 2001 11:18 am

Alright all you smart people:

Regarding related rates,

Assume x and y are both differentiable functions of t and find the required values of dy/dt and dx/dt .

Equation: xy = 4

Find: dy/dt when x = 8
Given: dx/dt = 10

and

Find: dx/dt when x = 1
Given: dy/dt = -6

Please someone shed some light!

Matt
Sometimes I go about in pity for myself and all the while a great wind carries me across the sky.
 
PanAm747
Posts: 4713
Joined: Mon Mar 01, 2004 4:46 am

RE: Calculus Help!

Fri Oct 12, 2001 11:40 am

you have to use the multiplication rule:

xy'+yx'=4

I think, it's been a while.
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FlyVirgin744
Topic Author
Posts: 1282
Joined: Tue Jul 27, 1999 8:35 am

RE: Calculus Help!

Fri Oct 12, 2001 11:51 am

Panam, that rule does come into play but it gets quite a bit more complicated. Thank you for your response though.  Smile
Sometimes I go about in pity for myself and all the while a great wind carries me across the sky.
 
pgh234
Posts: 732
Joined: Tue Jul 10, 2001 12:48 pm

RE: Calculus Help!

Fri Oct 12, 2001 12:22 pm

OMG, that is SOOOOOO creepy. I am doing the EXACT SAME PROBLEM for my AP Calculus homework as I am reading your post. Do you happen to have the same book as me also? It is a red Calculus book with "Larson-Hostetler-Edwards" Second edition on the bottom of the cover and the problem you are refering to is on Page 178, problem #3a in section 2.7?

anyways, to solve:

xy = 4 dx/dt=10 and find dy/dt when X=8

(x dy/dt + y dx/dt) = 0 (Multiplication Rule as stated above)
dy/dt = -(y/x)dx/dt
dy/dt = - ((1/2)/8) * 10 (Found Y value by solving equation for X=8)
dy/dt = -5/8

Good Luck in Calculus, cuz so far, it sucks!
pgh234
 
FlyVirgin744
Topic Author
Posts: 1282
Joined: Tue Jul 27, 1999 8:35 am

RE: Calculus Help!

Fri Oct 12, 2001 12:30 pm

Thank you so much! Seems so easy now  Wink/being sarcastic

I am actually using the 6th edition of your book! Its brand new, could have some better examples though.

Well thanx again and good luck to you also in your class. And oh yes, it definetely does suck.
Sometimes I go about in pity for myself and all the while a great wind carries me across the sky.

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